U.S. Pat. No. 5,432,015, issued Jul. 11, 1995, to Wu et al., and U.S. Pat. No. 5,756,147, issued May 26, 1998, to Wu et al. disclose an electroluminescent laminate structure which combines a thick film dielectric layer with thin film layers, and a rear to front method of forming same on a rigid, rear substrate. Solid state displays (SSD) using this hybrid thick film/thin film technology have been demonstrated to have good performance and brightness (luminosity) in monochrome (ZnS:Mn phosphor) and full colour n(ZnS:Mn/SrS:Ce bilayer phosphor) applications (Bailey et al., SID 95 Digest, 1995), however, improvements are still needed.
The potential for EL as a competitive alternative for fabricating flat panel displays has been hindered by the inability to generate bright, stable full colour. This has resulted in EL only penetrating markets for niche applications, in which the inherent benefits of the technology, such as ruggedness, wide viewing angle, temperature insensitivity, and fast time response, are needed.
Two basic alternatives have been used to produce full colour EL devices. One approach is to use patterned phosphors, that is alternating red, green and blue (RGB) phosphor elements in a layer (see for example U.S. Pat. No. 4,977,350, issued Dec. 11, 1990, to Tanaka et al.). This approach has the disadvantage of requiring the three phosphors to be patterned into red, green and blue sub-pixels that make up each pixel, in separate steps. Furthermore, the three colours cannot all be produced brightly enough by currently available EL phosphors to gain the brightness advantage desired. A second approach is to use a colour by white technique, first described by Tanaka et al., (SID 88 Digest, p 293, 1988, see also, U.S. Pat. No. 4,727,003, issued Feb. 23, 1988 to Ohseto et al.). In the colour by white method, the phosphor layer comprises layers of phosphors, typically ZnS:Mn and SrS:Ce, which when superimposed produce white light. Red, green and blue sub-pixels are then obtained by placing a patterned filter in front of the white light. The white phosphor emits light at wavelengths over the entire visible portion of the electromagnetic spectrum, and the filters transmit a narrowed range of wavelengths corresponding to the colours for each sub-pixel. This approach has the disadvantage of relatively poor energy efficiency, in high measure because a high fraction of the light is absorbed in the filters and the overall energy efficiency of the display is correspondingly reduced.
Another requirement for full colour displays is gray scale capability, that is the ability to generate a number of defined and consistent luminosities (light emission intensities) for each sub-pixel. Typically, 256 gray scale luminosities span a range from zero to full luminosity controlled by predetermined input electrical signals for each sub-pixel. This number of gray levels provides a total of about 16 million individual colours.
Electroluminescent displays have pixels and sub-pixels that are defined by intersecting sets of conductor stripes at right angles to one another on opposite sides of a phosphor layer. These sets of stripes are respectively referred to as “rows” and “columns”. The sub-pixels are independently illuminated using an addressing scheme called passive matrix addressing. This entails sequentially addressing the rows by applying a short flat-topped electrical pulse with a peak voltage called the threshold voltage sequentially on each of the rows such that the duration of the pulse is less than the time allocated for addressing each row. Electrical pulses, each with a defined and independent peak voltage, termed the “modulation voltage”, are simultaneously applied to each of the columns intersecting the addressed row. This provides independently controllable voltages across the sub-pixels making up the pixels along that row, in accordance with the instantaneous luminosity required for each sub-pixel to achieve the desired pixel colours. While each row is being addressed, the remaining rows are disconnected, or are connected to a voltage level near zero. Independent operation of all sub-pixels on the display requires that sub-pixels not on the addressed row do not illuminate. The electro-optical characteristics of the sub-pixels on an electroluminescent display facilitate meeting this requirement, by virtue of the fact that no luminosity is generated if the voltage across the sub-pixels is below the threshold voltage.
The time required to address all the rows in a display is called a frame, and for video images, the frame repetition rate must be at least about 50 Hz in order to avoid image flicker. At the same time there is a maximum frame repetition rate, typically about 200 Hz, that is achievable due to a limitation on the voltage rise time associated with the electrical characteristics of the display and its associated electronics. In principle, a measure of gray scale can be achieved by controlling the average pixel luminosity by modulating the average frame rate. This requires omitting a fraction of the electrical pulses over a suitably short period of time. In practice, however, due to the limited range of frame rates, only a few levels of gray scale can be realized this way. Another option, called dithering, is to extinguish one or more pixels in the immediate vicinity of a pixel where reduced luminosity is required, thereby spatially modulating luminosity. This technique, however, causes a loss of display resolution and image quality.
The preferred method of gray scale control is to control the instantaneous sub-pixel luminosity, which must be done by modulating the electrical pulse peak voltage, pulse duration or pulse shape. At the same time, to minimize power consumption in electroluminescent displays addressed using passive matrix addressing, it is desirable to have the row voltage as close as possible to the threshold voltage above which luminosity is generated. This requires the threshold voltage for all sub-pixels to be equal.
Filters used to tailor the spectral emission characteristics of sub-pixels typically do not have ideal characteristics. They do not have perfect transmission in the desired wavelength ranges to achieve the desired red, green and blue colours, and they have some optical transparency in the wavelength ranges where they should be opaque. These deviations from ideal behavior impose design limitations on the overall pixel design. For example, the polymer based blue filters commonly used for electroluminescent and other types of flat panel displays have some transmission also in the red portion of the spectrum. The need to suppress red contamination of the blue pixel requires that thicker polymer films be used, which reduces the transparency in the desired blue wavelength range. They also have some transparency in the green wavelength range introducing a similar requirement for thicker polymers that are less transparent to blue light. To meet the requirements for full colour displays, the ratios of luminosity for red:green:blue sub-pixels should be 3:6:1, to give a white colour for that pixel. The CIE colour coordinates for red sub-pixels should be in the range 0.60<x<0.65 and 0.34<y<0.36. The CIE colour coordinates for green sub-pixels should be in the range 0.35<x<0.38 and 0.55<y<0.62. For blue sub-pixels the CIE colour coordinates should be in the range 0.13<x<0.15 and 0.14<y<0.18. The combined (white) luminosity for a pixel comprising red, green and blue sub-pixels should be at least about 70 candelas per square meter (cd/m2 ) and the CIE colour coordinates for full white should be in the range 0.35<x<0.40 and 0.35<y<0.40. Higher luminosity is desirable for some applications.
Phosphors useful in electroluminescent displays are well known, and consist of a host material and an activator or dopant. The host material is usually a compound of a Group II element of the periodic table, with a Group VI element, or is a thiogallate compound. Examples of typical phosphors include zinc sulfide or strontium sulfide, with a dopant or activator which functions as the luminescent center when an electric field is applied across the phosphor. Typical activators with phosphors based on zinc sulfide include manganese (Mn) for an amber emission, terbium (Tb) for a green emission and samarium (Sm) for a red emission. A typical activator with phosphors based on strontium sulfide is Ce for a blue-green emission. It is conventional to refer to phosphors as, for example, SrS:Ce to designate a phosphor based on SrS doped with Ce, and ZnS:Mn to designate a phosphor based on ZnS doped with Mn, and this convention is used herein. It is also conventional, when using the formula for the phosphor, for example as in ZnS, to mean phosphors which are formed predominantly from a stoichiometric zinc sulfide. Other elements might be included in the host material for the phosphor, however it is typically still referred to as a phosphor based on the predominant component of the host material. Thus for instance when referring to a phosphor based on zinc sulfide, or a zinc sulfide phosphor, the terminology includes both pure zinc sulfide as a host material and, for example, the phosphor Zn1−xMgxS:Mn (designating a phosphor based on zinc sulfide but also including magnesium sulfide in the zinc sulfide host material, doped with Mn), although it is also understood that ZnS and Zn1−xMgxS are different host materials. This phosphor terminology is used herein and the patent claims.